Table of contents

Over the past few decades, impressive progress has been made in the field of photon polarimetry, especially in the hard X-ray and soft gamma-ray energy regime. Measurements of the linear degree of polarization for some of the most energetic astrophysical sources, such as gamma-ray bursts (GRBs) or blazars, are now possible, at energies below the pair creation threshold. As such, a new window has been opened for understanding the exact nature of the nonthermal emission mechanisms responsible for some of the most energetic phenomena in the universe. There are still many open questions and active debates, such as the discrimination between leptonic versus hadronic models of emission for Blazars or ordered versus random field models for GRBs. Because the competing models predict different levels of linear photon polarization at energies above ∼1 MeV, gamma-ray polarimetry in that energy band could provide additional crucial insights. However, no polarimeter for gamma rays with energies above ∼1 MeV has been flown into space, as the sensitivity is severely limited by a quick degradation of the angular resolution and by multiple Coulomb scatterings in the detector. Over the past few years, a series of proposals and demonstrator instruments that aim to overcome those inherent difficulties have been put forth, and the prospects look promising. The paper is organized as follows: Section 1, briefly reviews the history and principles of gamma-ray polarimetry, emphasizing its challenges and successes; Section 2 is dedicated the discussion of gamma-ray polarization and polarimetry, and Section 3 discusses the past and current instruments with which measurements of linear polarization for hard X-rays and soft gamma rays were successfully obtained for astrophysical sources; Section 4 outlines the scientific questions that could be solved by using gamma-ray polarimetry measurements. A summary and outlook are provided in Section 5.

Far-ultraviolet (FUV) GALEX photometry of main-sequence stars in moving groups is used to verify that there is a relationship between age and FUV broadband magnitude among such stars. Samples of dwarf stars were compiled for twelve moving groups and a search made for both GALEX FUV magnitudes on the AB system and Johnson BV photometry. These data sets were used to calculate a (FUV − B) color index for each star, which in turn was plotted against B − V in a two-color diagram. Based on the loci that the group dwarfs define in this two-color space, there is strong evidence that among candidate members of the Hyades, HR 1614, Wolf 630, Ursa Major, Arcturus, AB Doradus, η Cephei, β Pictoris, and Tucana–Horologium moving groups there are populations of coeval stars. Evidence for coevality is weaker in the case of the ζ Herculis group, and no conclusion can be drawn for candidate stars that have previously been assigned to the Kapteyn's Star and σ Puppis groups. Polynomials of up to fifth order were fitted to the two-color loci of the dwarfs in each group. These fits were used to calculate fiducial values of (FUV − B) throughout a range in B − V color on the main sequence for most of the groups studied here. Evidence is found that at a fixed B − V there is a logarithmic dependence of (FUV − B) color on stellar age for FGK dwarfs. The relationships presented herein could potentially serve as a tool for estimating the ages of late-type Population I dwarf stars.

We build H i absorption spectra toward Supernova Remnant (SNRs) G16.7+0.1 and G15.9+0.2 using the THOR survey data. With the absorption spectra, we give a new distance range of 7–16 kpc for G15.9+0.2. We also resolve the near/far-side distance ambiguity of G16.7+0.1 and confirm its kinematic distance of about 14 kpc. In addition, we analyze the CO (J = 3–2) spectra toward G16.7+0.1 and find obvious CO emission at the 20 km s⁻¹ OH 1720 MHz maser site. This supports Reynoso & Mangum suggestions that the velocity difference between the maser and southern molecular cloud is caused by the shock acceleration. We discuss the impact of the distances on other physical parameters of the two SNRs.

We present design considerations for a ground-based survey for transiting exoplanets around L and T dwarfs, spectral classes that have yet to be thoroughly probed for planets. We simulate photometry for L and T targets with a variety of red-optical and near-infrared (NIR) detectors, and compare the scatter in the photometry to anticipated transit depths. Based on these results, we recommend the use of a low-dark-current detector with H-band NIR photometric capabilities. We then investigate the potential for performing a survey for Earth-sized planets for a variety of telescope sizes. We simulate planetary systems around a set of spectroscopically confirmed L and T dwarfs using measured M dwarf planet occurrence rates from Kepler, and simulate their observation in surveys ranging in duration from 120 to 600 nights, randomly discarding 30% of nights to simulate weather losses. We find that an efficient survey design uses a 2 m class telescope with a NIR instrument and 360–480 observing nights, observing multiple L and T targets each night with a dithering strategy. Surveys conducted in such a manner have over an 80% chance of detecting at least one planet, and detect around 2 planets, on average. The number of expected detections depends on the true planet occurrence rate, however, which may in fact be higher for L and T dwarfs than for M dwarfs.

The K2 mission has enabled searches for transits in crowded stellar environments very different from the original Kepler mission field. We describe here the reduction and analysis of time series data from K2's Campaign 0 superstamp, which contains the 150 Myr open cluster M35. We report on the identification of a substellar transiting object orbiting an A star at the periphery of the superstamp. To investigate this transiting source, we performed ground based follow-up observations, including photometry with the Las Cumbres Observatory telescope network and high resolution spectroscopy with Keck/High Resolution Echelle Spectrometer. We confirm that the host star is a hot, rapidly rotating star, precluding precision radial velocity measurements. We nevertheless present a statistical validation of the planet or brown dwarf candidate using speckle interferometry from the WIYN telescope to rule out false positive stellar eclipsing binary scenarios. Based on parallax and proper motion data from Gaia Data Release 2 (DR2), we conclude that the star is not likely to be a member of M35, but instead is a background star around 100 pc behind the cluster. We present an updated ephemeris to enable future transit observations. We note that this is a rare system as a hot host star with a substellar companion. It has a high potential for future follow-up, including Doppler tomography and mid-infrared secondary transit observations.

Sodium laser guide star (LGS) adaptive optics (AO) systems have become highly productive tools for all-sky astronomical observations with large telescopes. However, the performance of these systems is limited by the brightness of the sodium LGS and the number of wavefront sensor subapertures used during strong atmospheric turbulence. We propose a hybrid pulsed Rayleigh-sodium LGS AO system that exhibits an improved Strehl ratio of 55% to eliminate the effects of partial wavefront sampling for limited sodium LGS brightness in K-band. This technique uses the Rayleigh scattering signal, a byproduct of pulsed sodium LGS, with temporal-gating approach. This system comprises a Rayleigh LGS to direct light toward the wavefront sensor under a wide variety of atmospheric conditions. The Rayleigh beacon provides high-order corrections to the low-altitude layers. Any remaining distortions from higher layers typically exist on a much larger scale, which can be sampled with comparatively larger subapertures. This approach could improve the performance of large optics-infrared telescopes equipped with hybrid Rayleigh-sodium LGS AO systems during strong turbulence events.

We analyze the postdoctoral career tracks of a nearly complete sample of astronomers from 28 United States graduate astronomy and astrophysics programs spanning 13 graduating years ($N=1063$). A majority of both men and women (65% and 66%, respectively) find long-term employment in astronomy or closely related academic disciplines. We find no significant difference in the rates at which men and women are hired into these jobs following their Ph.D.s or in the rates at which they leave the field. Applying a two-outcome survival analysis model to the entire data set, we measure a relative academic hiring probability ratio for women versus men at a common year -post-Ph.D. of ${H}_{F/M}={1.08}_{-0.17}^{+0.20}$ and a leaving probability ratio of ${L}_{F/M}={1.03}_{-0.24}^{+0.31}$ (95% CI). These are both consistent with equal outcomes for both genders (${H}_{F/M}={L}_{F/M}=1$) and rule out more than minor gender differences in hiring or in the decision to abandon an academic career. They suggest that despite discrimination and adversity, women scientists are successful at managing the transition between Ph.D., postdoctoral, and faculty/staff positions.

Distant starlight passing through Earth's atmosphere is refracted by an angle of just over one degree near the surface. This focuses light onto a focal line starting at an inner (and chromatic) boundary out to infinity, offering an opportunity for pronounced lensing. It is shown here that the focal line commences at ∼85% of the Earth–Moon separation, thus placing an orbiting detector between here and one Hill radius could exploit this refractive lens. Analytic estimates are derived for a source directly behind Earth (i.e., on-axis) showing that starlight is lensed into a thin circular ring of thickness, WH_Δ/R, yielding an amplification of 8H_Δ/W, where H_Δ is Earth's refractive scale height, R is its geopotential radius, and W is the detector diameter. These estimates are verified through numerical ray-tracing experiments from optical to 30 μm light with standard atmospheric models. The numerical experiments are extended to include extinction from both a clear atmosphere and one with clouds. It is found that a detector at one Hill radius is least affected by extinction, as lensed rays travel no deeper than 13.7 km, within the statosphere and above most clouds. Including extinction, a 1-m Hill radius "terrascope" is calculated to produce an amplification of ∼45,000 for a lensing timescale of ∼20 hr. In practice, the amplification is likely halved to avoid daylight scattering i.e., 22,500 (Δmag = 10.9) for W = 1 m, or equivalent to a 150 m optical/infrared telescope.

The James Webb Space Telescope (JWST) will observe several stars for long cumulative durations while pursuing exoplanets as primary science targets for both Guaranteed Time Observations (GTO) and very likely General Observer (GO) programs. Here we argue in favor of an automatic default parallel program to observe, e.g., using the F200W/F277W filters or grism of NIRCAM/NIRISS in order to find high redshift (z ≫ 10) galaxies, cool red/brown dwarf substellar objects, solar system objects, and observations of serendipitous planetary transits. We argue here the need for automated exploratory astrophysical observations with unused JWST instruments during these long-duration exoplanet observations. Randomized fields that are observed in parallel mode reduce errors due to cosmic variance more effectively than single continuous fields of a typical wedding cake observing strategy. Hence, we argue that the proposed automated survey will explore a unique and rich discovery space in the high-redshift universe, Galactic structure, and solar system. We show that the GTO and highly probable GO target list of exoplanets covers the Galactic disk/halo and high redshift universe, mostly well out of the plane of the disk of the Milky Way. Exposure times are of the order of the CEERS GTO medium-deep survey in a single filter, comparable to CANDELS in Hubble Space Telescope's surveys and deep fields. The area covered by NIRISS and NIRCam combined could accumulate to a half square degree surveyed.

This paper proposes a new approach to free-form cluster lens modeling that is inspired by the JPEG image compression method. This approach is motivated specifically by the need for accurate modeling of high-magnification regions in galaxy clusters. Existing modeling methods may struggle in these regions due to their limited flexibility in the parameterization of the lens, even for a wide variety of free-form methods. This limitation especially hinders the characterization of faint galaxies at high redshifts, which have important implications for the formation of the first galaxies and even for the nature of dark matter. JPEG images are extremely accurate representations of their original, uncompressed counterparts but use only a fraction of number of parameters to represent that information. Its relevance is immediately obvious to cluster lens modeling. Using this technique, it is possible to construct flexible models that are capable of accurately reproducing the true mass distribution using only a small number of free parameters. Transferring this well-proven technology to cluster lens modeling, I demonstrate that this "JPEG parameterization" is indeed flexible enough to accurately approximate an N-body simulated cluster.

We present an algorithm that uses the distribution of photon arrival times to distinguish speckles from incoherent sources, like planets and disks, in high-contrast images. Using simulated data, we show that our approach can overcome the noise limit from fluctuating speckle intensity. The algorithm is likely to be most advantageous when a coronagraph limits the coherent diffraction pattern in the image plane but the intensity is still strongly modulated by fast-timescale uncorrected stellar light, for example from atmospheric turbulence. These conditions are common at small inner working angles of highly corrected adaptive optics images and will allow probing of exoplanet populations at smaller angular separations. The technique requires a fast science camera that can temporally resolve the speckle fluctuations, and the detection of many photons per speckle decorrelation time. Because the algorithm directly extracts the incoherent light, standard differential imaging postprocessing techniques can be performed afterwards to further boost the signal.

We introduce a new pipeline for analyzing and mitigating radio frequency interference (RFI), which we call Sky-Subtracted Incoherent Noise Spectra (ssins). ssins is designed to identify and remove faint RFI below the single baseline thermal noise by employing a frequency-matched detection algorithm on baseline-averaged amplitudes of time-differenced visibilities. We demonstrate the capabilities of ssins using the Murchison Widefield Array (MWA) in Western Australia. We successfully image aircraft flying over the array via digital television reflection detected using ssins and summarize an RFI occupancy survey of MWA Epoch of Reionization data. We describe how to use ssins with new data using a documented, publicly available implementation with comprehensive usage tutorials.

In 2018, Solar Cycle 24 entered a deep solar minimum. During this period, we collected night sky brightness data at Cosmic Campground International Dark Sky Sanctuary (CCIDSS) in the USA (2018 September 4–2019 January 4) and at Aotea/Great Barrier Island International Dark Sky Sanctuary (AGBIIDSS) in New Zealand (2018 March 26–August 31. These sites have artificial-light-pollution-free natural night skies. The equipment employed are identical Unihedron SQM-LU-DL meters, used as single-channel differential photometers, to scan the sky as Earth rotates on its axis. We have developed new analysis techniques which select those data points which are uninfluenced by Sun, Moon, or clouds to follow brightness changes at selected points on the celestial sphere and to measure the brightness of the airglow above its quiescent level. The 2018 natural night sky was measured to change in brightness by approximately 0.9 mag arcsec⁻² at both locations. Preliminary results indicate the modulations of the light curves (brightness versus R.A.) we observed are related in complex ways to elements of space weather conditions in the near-Earth environment. In particular, episodes of increased night sky brightness are observed to be contemporaneous with geomagnetic activity, increases in mean solar wind speed, and some solar proton/electron fluence events. Charged particles in the solar wind take days to reach near-Earth environment after a coronal hole is observed to be facing in our direction. Use of this information could make it possible to predict increases in Earth's natural night sky brightness several days in advance. What we have learned during this solar minimum leads us to search for other solar driven changes in night sky brightness as the Sun begins to move into solar maximum conditions.

The MINiature Exoplanet Radial Velocity Array (MINERVA) is a dedicated observatory of four 0.7 m robotic telescopes fiber-fed to a KiwiSpec spectrograph. The MINERVA mission is to discover super-Earths in the habitable zones of nearby stars. This can be accomplished with MINERVA's unique combination of high precision and high cadence over long time periods. In this work, we detail changes to the MINERVA facility that have occurred since our previous paper. We then describe MINERVA's robotic control software, the process by which we perform 1D spectral extraction, and our forward modeling Doppler pipeline. In the process of improving our forward modeling procedure, we found that our spectrograph's intrinsic instrumental profile is stable for at least nine months. Because of that, we characterized our instrumental profile with a time-independent, cubic spline function based on the profile in the cross dispersion direction, with which we achieved a radial velocity precision similar to using a conventional "sum-of-Gaussians" instrumental profile: 1.8 m s⁻¹ over 1.5 months on the RV standard star HD 122064. Therefore, we conclude that the instrumental profile need not be perfectly accurate as long as it is stable. In addition, we observed 51 Peg and our results are consistent with the literature, confirming our spectrograph and Doppler pipeline are producing accurate and precise radial velocities.

Data challenges are emerging as powerful tools with which to answer fundamental astronomical questions. Time-domain astronomy lends itself to data challenges, particularly in the era of classification and anomaly detection. With improved sensitivity of wide-field surveys in optical and radio wavelengths from surveys like the Large Synoptic Survey Telescope (LSST) and the Canadian Hydrogen Intensity Mapping Experiment, we are entering the large-volume era of transient astronomy. We highlight some recent time-domain challenges, with particular focus on the Photometric LSST Astronomical Time series Classification Challenge, and describe metrics used to evaluate the performance of those entering data challenges.

We present Real-time Automated Photometric IDentification (RAPID), a novel time series classification tool capable of automatically identifying transients from within a day of the initial alert, to the full lifetime of a light curve. Using a deep recurrent neural network with gated recurrent units (GRUs), we present the first method specifically designed to provide early classifications of astronomical timeseries data, typing 12 different transient classes. Our classifier can process light curves with any phase coverage, and it does not rely on deriving computationally expensive features from the data, making RAPID well suited for processing the millions of alerts that ongoing and upcoming wide-field surveys such as the Zwicky Transient Facility (ZTF), and the Large Synoptic Survey Telescope (LSST) will produce. The classification accuracy improves over the lifetime of the transient as more photometric data becomes available, and across the 12 transient classes, we obtain an average area under the receiver operating characteristic curve of 0.95 and 0.98 at early and late epochs, respectively. We demonstrate RAPID's ability to effectively provide early classifications of observed transients from the ZTF data stream. We have made RAPID available as an open-source software package⁸for machine-learning-based alert brokers to use for the autonomous and quick classification of several thousand light curves within a few seconds.

Telescope scheduling is the task of determining the best sequence of observations (pointings and filter choices) for a survey system. Because it is computationally intractable to optimize over all possible multiyear sequences of observations, schedulers use heuristics to pick the best observation at a given time. A greedy scheduler selects the next observation by choosing whichever one maximizes a scalar merit function, which serves as a proxy for the scientific goals of the telescope. This sort of bottom-up approach for scheduling is not guaranteed to produce a schedule for which the sum of merit over all observations is maximized. As an alternative to greedy schedulers, we introduce ALTSched, which takes a top-down approach to scheduling. Instead of considering only the next observation, ALTSched makes global decisions about which area of sky and which filter to observe in, and then refines these decisions into a sequence of observations taken along the meridian to minimize airmass. We implement ALTSched for the Large Synoptic Survey Telescope (LSST) and show that it equals or outperforms the baseline greedy scheduler in essentially all quantitative performance metrics. Due to its simplicity, our implementation is considerably faster than OpSim, the simulated greedy scheduler currently used by the LSST Project: a full 10-yr survey can be simulated in 4 minutes, as opposed to tens of hours for OpSim. LSST's hardware is fixed, so improving the scheduling algorithm is one of the only remaining ways to optimize LSST's performance. We see ALTSched as a prototype scheduler that gives a lower bound on the performance achievable by LSST.

The Minerva-Australis telescope array is a facility dedicated to the follow-up, confirmation, characterization, and mass measurement of planets orbiting bright stars discovered by the Transiting Exoplanet Survey Satellite (TESS)—a category in which it is almost unique in the Southern Hemisphere. It is located at the University of Southern Queensland's Mount Kent Observatory near Toowoomba, Australia. Its flexible design enables multiple 0.7 m robotic telescopes to be used both in combination, and independently, for high-resolution spectroscopy and precision photometry of TESS transit planet candidates. Minerva-Australis also enables complementary studies of exoplanet spin–orbit alignments via Doppler observations of the Rossiter–McLaughlin effect, radial velocity searches for nontransiting planets, planet searches using transit timing variations, and ephemeris refinement for TESS planets. In this first paper, we describe the design, photometric instrumentation, software, and science goals of Minerva-Australis, and note key differences from its Northern Hemisphere counterpart, the Minerva array. We use recent transit observations of four planets, WASP-2b, WASP-44b, WASP-45b, and HD 189733b, to demonstrate the photometric capabilities of Minerva-Australis.

We propose a polarimeter, which is dedicated to Earth-like exoplanet imaging for future space missions. We adopt a minimum-polarization-component design philosophy, which makes a compact and robust system as well as high-performance achievable in the real world. Our polarimeter consists of two polarization components of a liquid crystal variable retarders (LCVR) and a Wollaston prism. The polarimeter can deliver an extra contrast better than 10^−4.5. Combined with one of the currently available coronagraphs that are delivering a contrast on the order of 10^−6.5, the coronagraph and polarimeter system can deliver a contrast better than 10⁻¹¹ at a small inner working angle in the visible over the entire imaging plane. We discuss the polarimeter design concept and dedicated data-reduction technique. Our unique host-star calibration algorithm allows the starlight to be totally removed, regardless of whether the host-star image is intrinsically polarized or whether the light is polarized by preoptics, such as the telescope that is located before the polarimeter, which makes exoplanet polarization imaging feasible with any telescope, optical system, and target star. Using minimum-polarization components with a solid-state image LCVR as the key polarization component, our polarimeter is less sensitive to the wavefront phase and amplitude errors than other exoplanet imaging techniques. Based on commercial-grade optical components, we demonstrated for the first time that by combining our polarimeter with a currently available coronagraph, the polarimeter and coronagraph system can deliver a contrast better than 10⁻¹¹ at a small inner working angle in the visible wavelengths, which paves the way for Earth-like exoplanet imaging for a future space mission.